System and method for monitoring reliability of a digital system

ABSTRACT

System and method are provided for continually monitoring reliability, or aging, of a digital system and for issuing a warning signal if digital system operation degrades past a specified threshold. The technique includes implementing a ring oscillator sensor in association with the digital system, wherein logic and/or device percent composition of the ring oscillator sensor mirrors percent composition thereof within the digital system. Counter logic is coupled to the ring oscillator sensor for converting outputted count signals to an oscillation frequency, and control logic is coupled to the counter logic for periodically evaluating oscillation frequency of the ring oscillator sensor and generating a warning signal indicative of reliability degradation if at least one of: (i) a measured or estimated oscillation frequency is below a warning threshold frequency; or (ii) a measured or estimated rate of change in a difference between measured oscillation frequencies exceeds an acceptable rate of change threshold.

TECHNICAL FIELD

The present invention relates in general to the field of failureprediction and, more specifically, to a ring oscillator sensor-basedreliability measurement system and method for a digital system.

BACKGROUND OF THE INVENTION

Failure rates of individual components making up a digital system suchas an integrated circuit (or larger system) are fundamentally related tovarious parameters, including operating temperatures, as well as scalingof the digital system and interconnect geometries. Although burn-intesting of digital systems attempts to predict a lifecycle for a giventype of digital system, it does not provide aging information for eachspecific digital system of the type being manufactured. Currently, acustomer or user may uncover a problem with a digital system only aftera catastrophic system failure. While catastrophic failure of a digitalsystem is readily recognizable, a “soft” failure (where there may besignificant degradation in digital system performance or reliability)may go unnoticed, which implies that such aging of the digital systemmay cause undetected errors in computation and data, from which it isdifficult to recover.

SUMMARY OF THE INVENTION

Presented herein is an approach for actively monitoring or estimatingaging, and hence reliability, of a specific digital system and forissuing a warning signal if, for example, degradation of operationthereof, or more particularly, of an associated ring oscillator sensor,exceeds a specified threshold.

Thus, in one aspect, a system for monitoring reliability of a digitalsystem is provided. This system includes: at least one ring oscillatorsensor implemented in association with the digital system forfacilitating monitoring reliability thereof, wherein the digital systemincludes a circuit composition comprising at least one logic type, theat least one logic type comprising at least one device type. The atleast one ring oscillator sensor includes a circuit compositioncomprising one or more of the at least one logic type or the at leastone device type selected based on the circuit composition of the digitalsystem to correlate aging of the at least one ring oscillator sensor toaging of the digital system. The at least one ring oscillator sensoroutputs count signals, and the system further includes counter logic andcontrol logic. The counter logic is coupled to the at least one ringoscillator sensor for converting the count signals to an oscillationfrequency, while the control logic is coupled to the counter logic forperiodically evaluating oscillation frequency of the at least one ringoscillator sensor and generating a warning signal indicative ofreliability degradation thereof, and hence of the digital system, if atleast one of: (i) a measured or estimated oscillation frequency of theat least one ring oscillator sensor is below a warning thresholdfrequency for the digital system; or (ii) a measured or estimated rateof change in a difference between measured oscillation frequencies ofthe at least one ring oscillator sensor exceeds an acceptable rate ofchange threshold for the digital system.

In another aspect, a system for monitoring reliability of a digitalsystem is provided which includes at least one ring oscillator sensorembedded within the digital system for facilitating monitoringreliability thereof. The digital system includes a circuit compositioncomprising at least one logic type and at least one device type employedwithin the at least one logic type. The at least one ring oscillatorsensor includes a circuit composition at least partially mirroringpercentages of the at least one logic type and the at least one devicetype employed in the circuit composition of the digital system, whereinaging of the at least one ring oscillator sensor is correlated to agingof the digital system. The at least one ring oscillator sensor outputscount signals, and the system further includes counter logic and controllogic. The counter logic is coupled to the at least one ring oscillatorsensor for converting count signals to an oscillation frequency, whilethe control logic is coupled to the counter logic for periodicallyevaluating oscillation frequency of the at least one ring oscillatorsensor and generating a warning signal indicative of reliabilitydegradation thereof, and hence of the digital system, if at least oneof: (i) a measured or estimated oscillation frequency of the at leastone ring oscillator sensor is below a warning threshold frequency forthe digital system; or (ii) a measured or estimated rate of change in adifference between measured oscillation frequencies of the at least onering oscillator sensor exceeds an acceptable rate of change thresholdfor the digital system.

In a further aspect, a method of monitoring reliability of a digitalsystem is provided. This method includes: obtaining at least one ringoscillator sensor embedded within a digital system for facilitatingmonitoring reliability thereof, the digital system including a circuitcomposition comprising at least one logic type, the at least one logictype comprising at least one device type, and wherein logic and devicetype composition percentages for the at least one ring oscillator sensormirror circuit composition percentages of one or more of the at leastone logic type or the at least one device type within the digitalsystem, thereby correlating aging of the at least one ring oscillatorsensor to aging of the digital system, the at least one ring oscillatorsensor outputting count signals; converting the count signals of the atleast one ring oscillator sensor to an oscillation frequency of the atleast one ring oscillator sensor; and periodically evaluatingoscillation frequencies of the at least one ring oscillator sensor, andgenerating a warning signal indicative of a reliability degradationthereof, and hence of the digital system, if at least one of: (i) ameasured or estimated oscillation frequency of the at least one ringoscillator sensor is below a warning threshold frequency for the digitalsystem; or (ii) a measured or estimated rate of change in a differencebetween measured oscillation frequencies of the at least one ringoscillator sensor exceeds an acceptable rate of change threshold for thedigital system.

Further, additional features and advantages are realized through thetechniques of the present invention. Other embodiments and aspects ofthe invention are described in detail herein and are considered a partof the claimed invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 depicts one embodiment of a monitoring system, in accordance withan aspect of the present invention;

FIG. 2 depicts one embodiment of a digital system employing a monitoringsystem assimilating device types of the digital system within the ringoscillator sensor, in accordance with an aspect of the presentinvention;

FIGS. 2A-2D depict various device type examples for inverter logicemployable in a digital system to be monitored, in accordance with anaspect of the present invention;

FIG. 3 depicts an alternate embodiment of a digital system employing amonitoring system assimilating logic types of the digital system withinthe ring oscillator sensor, in accordance with an aspect of the presentinvention;

FIG. 4 depicts a further embodiment of a digital system employing amonitoring system assimilating both logic and device types of theobserved digital block within the ring oscillator sensor thereof, inaccordance with an aspect of the present invention;

FIG. 5 depicts a digital system comprising multiple digital functionblocks, each having an associated ring oscillator sensor of a monitoringsystem, wherein each ring oscillator sensor assimilates percentages oflogic types and device types of the digital system, in accordance withan aspect of the present invention;

FIG. 6 depicts one embodiment of a digital function block havingmultiple circuit areas, each having an associated ring oscillator sensorof a monitoring system assimilating percentages of logic types anddevice types of the digital system, in accordance with an aspect of thepresent invention;

FIG. 7 depicts one embodiment of a digital function block having anassociated ring oscillator sensor of a monitoring system powered by asame power supply as the digital function block being monitored, inaccordance with an aspect of the present invention;

FIG. 8 depicts one embodiment of a digital system having multipledigital function blocks, each having an associated ring oscillatorsensor of a monitoring system powered by a same power supply as thedigital function block, in accordance with an aspect of the presentinvention;

FIG. 9 depicts one embodiment of a digital function block and anassociated ring oscillator sensor (of a monitoring system) which isenabled by a request received at the digital function block, inaccordance with an aspect of the present invention;

FIG. 10 depicts one embodiment of a digital system comprising multipledigital function blocks, each having an associated ring oscillatorsensor (of a monitoring system) which is enabled by a respective requestreceived at the digital function block being monitored, in accordancewith an aspect of the present invention;

FIG. 11 graphically depicts digital system and correlated ringoscillator sensor lifecycles, illustrating oscillation frequency agingfor two different ring oscillator sensors compared with aging of themaximum frequency of operation of the digital system, in accordance withan aspect of the present invention;

FIG. 12 is a flowchart of one embodiment of logic for trackingoscillation frequency of a ring oscillator sensor, in accordance with anaspect of the present invention;

FIG. 13 is a flowchart of one embodiment of logic for performingoscillation frequency trend analysis, and for generating a warningsignal based thereon, in accordance with an aspect of the presentinvention;

FIG. 14 is a flowchart of an alternate embodiment of logic forperforming oscillation frequency trend analysis, and for generating awarning signal based thereon, in accordance with an aspect of thepresent invention;

FIG. 15 is a flowchart of another embodiment of logic for performingoscillation frequency trend analysis, and for generating a warningsignal based thereon, in accordance with an aspect of the presentinvention.

FIG. 16 is a flowchart of one embodiment of logic for implementing avariable sampling period for evaluating oscillation frequency of a ringoscillator sensor, in accordance with an aspect of the presentinvention; and

FIG. 17 depicts one embodiment of a computer program product toincorporate one or more aspects of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As noted, presented herein are a monitoring system and method foractively tracking aging, and hence reliability, of a specific digitalsystem, and for issuing a warning signal if, for example, degradation ofthe monitoring system exceeds a pre-specified threshold. The “digitalsystem” refers to any digital system or circuit, and includes, forexample, a processor, as well as simple or complex non-processor baseddigital logic, memory, etc. As one specific example, the digital systemis a microprocessor, and the specified threshold is a predefinedacceptable level for the maximum frequency of operation of the digitalsystem.

More particularly, presented herein is a technique for monitoringreliability of a digital system employing one or more ring oscillatorsensors implemented in association with (e.g., embedded within) thedigital system. In one embodiment, the one or more ring oscillatorsensors are integrated into the digital system within available spacethereof. As a specific example, the digital system may comprise asemiconductor device, and the one or more ring oscillator sensors areintegrated into the semiconductor device adjacent to or within one ormore digital function blocks (or one or more circuit areas) of thedigital system to be monitored to facilitate correlation of aging of thering oscillator sensors with aging of the digital system.

Further, in accordance with an aspect of the present invention, the oneor more ring oscillator sensors of the monitoring system have a circuitcomposition comprising one or more logic types and/or device typesmirroring the circuit composition of the digital system to be monitored.As used herein “logic type” refers to a type of logic circuit such as anAND logic circuit, a NAND logic circuit, an OR logic circuit, a NORlogic circuit, or an INVERTER circuit. “Device type” refers to a type ofdevice used to implement a specific logic type. For example, thin oxidedevices, thick oxide devices, low VT-doped devices, or high VT-dopeddevices may be employed in implementing one or more logic types of thecircuit composition of the digital system. In one example, the devicetype refers to a transistor type, and includes one or more of thin oxidetransistors, thick oxide transistors, low VT-doped transistors and highVT-transistors.

As one example, if the circuit composition of the digital system to bemonitored comprises multiple logic types, with the multiple logic typesincluding 50% NAND logic circuits and 50% INVERTER logic circuits, thenthe ring oscillator sensor implemented in association with the digitalsystem includes a composition which mirrors the circuit composition ofthe digital system, that is, it includes 50% NAND logic circuits and 50%INVERTER logic circuits. Similarly, if the 50% NAND logic circuits ofthe circuit composition of the digital system are 100% thin oxidedevices, then the 50% NAND logic circuits in the ring oscillator sensorare also 100% thin oxide devices. In this manner, multiple device typesused in implementing the logic types of the digital system's circuitcomposition are also mirrored within the ring oscillator sensorimplemented in association with the digital system. This allows forbetter correlation between aging of the digital system and aging of thering oscillator sensor associated with the digital system. Further,oscillation frequency of the ring oscillator sensor is more closelytailored to the maximum frequency of operation of the digital systemsince different device types and logic types typically have differentspeeds of operation.

In addition to one or more ring oscillator sensors, the monitoringsystem and method presented herein employ counter logic and controllogic. The counter logic is coupled to the ring oscillator sensors forconverting count signals of a ring oscillator sensor to an oscillationfrequency. The control logic is coupled to the counter logic forperiodically evaluating oscillation frequency of the one or more ringoscillator sensors and for generating a warning signal indicative ofreliability degradation thereof, and hence of the digital system, if atleast one of: (i) a measured or estimated oscillation frequency of theat least one ring oscillator sensor is below a warning thresholdfrequency for the digital system; or (ii) a measured or estimated rateof change in a difference between measured oscillation frequencies ofthe at least one ring oscillator sensor exceeds an acceptable rate ofchange threshold for the digital system. In one implementation, thecounter logic and control logic are coupled to multiple ring oscillatorsensors implemented in association with the digital system. In alternateimplementations, each ring oscillator sensor may have its own associatedcounter logic and control logic performing the above-noted functions, inwhich case multiple separate monitoring systems would be implementedacross the digital system.

The above-noted and other aspects of the present invention are describedfurther below with reference to FIGS. 1-17.

Current techniques for monitoring aging of a digital system have anumber of drawbacks. Typically, there is an absence of physicaltransducers to directly sense and measure digital system aging, andsystem level aging detection detects only a “machine check” orapplication software error. There is no direct technique for measuringaging of a particular digital system. Further, there is no technique forwarning of an impending age related failure in a digital system, andthere is no technique available to avoid computational glitches arisingfrom a “soft aging” scenario. Digital system testing is conventionallyexpensive, time-consuming and not comprehensive.

In a typical digital system lifecycle model, the digital system has ahigher maximum frequency of operation (F_(MAX)) than a specified (i.e.,required) maximum frequency of operation for the digital system(F_(SPEC)) when manufactured and beginning its lifecycle. As the digitalsystem ages, several factors may degrade system performance, and hencedecrease maximum operating frequency as a result. Factors which degradedigital system performance depend upon the particular system at issueand the environment within which the system is used. For example, if thedigital system comprises a processor, aging can be caused by a varietyof factors, including hot election, electromigration and thermalexpansion of the digital system.

Two failure modes are possible. First, a hard failure is representativeof an abrupt failure of the digital system. Soft aging occurs whereoperation of the digital system gradually decreases to a level at orbelow the manufacturer specified minimum frequency of operation(F_(SPEC)). Due to the gradual nature of this aging, the soft agingfailure may go unnoticed, which implies that such aging may causeundetected errors in computation and data. Once the maximum frequency ofoperation of the digital system (F_(MAX)) is known to fall below themanufacturer specified maximum frequency of operation for the digitalsystem (F_(SPEC)) (meaning that the digital system fails to operate atthe required conditions), then the system must be replaced or repaired.Unfortunately, an accumulated aging effect with the system operating ator near the manufacturer specified maximum frequency of operation(F_(MAX)) might result in a single bit error in a block of data, makingit difficult to detect occurrence of such an error employing a testinstruction vector. This traditionally makes it difficult to distinguishthe boundary between good and bad data results in an aging digitalsystem.

FIG. 1 depicts one embodiment of a monitoring system to be implementedin association with a digital system for monitoring reliability of thedigital system. This monitoring system, generally denoted 100, includesone or more ring oscillator sensors 110, one or more drivers 119, one ormore counter logic blocks 120, and one or more control logic blocks 130.Each ring oscillator sensor 110 has a composition which typicallycomprises multiple components. In the embodiment of FIG. 1, ringoscillator sensor 110 includes a plurality of inverters 112 connectedin-series with a two-input NAND logic circuit 114. A first input to NANDcircuit 114 is an enable input 115, and a second input is a feedbacksignal 117 from the output of ring oscillator sensor 110. A common powersupply voltage VDD drives the components of the ring oscillator sensor.This ring oscillator sensor 110, when correlated to and implemented inassociation with a digital system, provides a circuit levelsensor/transducer for sensing digital processor aging.

As explained further below, by implementing the ring oscillator sensorin association with the digital system (e.g., in available space on asemiconductor die containing the digital system), aging of the ringoscillator sensor can be tailored to closely mirror or correlate toaging of the digital system. Ring oscillator sensor 110 is an analogring oscillator sensor, which is isolated from counter logic 120 viadriver 119. Counter logic 120 includes a frequency divider 122, whichfunctions as an analog-to-digital converter, and a counter register 124,which is optional and is employed to reduce the number of bits needed toimplement the counter logic. Correlation of the ring oscillatorsensor-to-digital system aging can be enhanced, as explained herein, byproviding (for example) multiple ring oscillator sensors associated withspecific digital function blocks of the digital system and/or multiplering oscillator sensors associated with different circuit areas of thedigital system, and by powering each ring oscillator sensor employing acommon power supply as used by the associated digital function block orcircuit area of the digital system, and enabling each ring oscillatorsensor only responsive to receipt of a request at the digital functionblock (or circuit area), thus ensuring the ring oscillator sensor isonly active when the corresponding digital function block (or circuitarea) is active. These and other aspects of the present invention aredescribed in detail below.

Advantageously, the ring oscillator sensor presented herein assimilatesdigital logic performance and hence the aging effect of the digitalsystem. The ring oscillator sensor is a simple circuit which is alow-power sensor and is employed in conjunction with high-accuracycounter logic. Control logic 130 implements one of a variety ofprocesses for periodically evaluating oscillation frequency of the oneor more ring oscillator sensors and for generating a warning signalindicative of reliability degradation thereof, and hence of the digitalsystem to which the ring oscillator system is correlated by design andoperation.

FIG. 2 depicts a simplified embodiment of a digital system 201comprising an observed or monitored digital function block 205, and amonitoring system 200 having a ring oscillator sensor 210 with a circuitcomposition which assimilates the device type composition of the digitalfunction block 205. As illustrated, digital function block 205 has acircuit composition which comprises one logic type, that is, an inverterlogic circuit. However, each inverter logic circuit is implemented usinga different device type. By way of example, FIGS. 2A-2D depict fourpossible device types for implementing an inverter logic circuit. InFIG. 2A, a thin oxide device is illustrated comprising a parallelconnected NFET and PFET pair. In FIG. 2B, a thick oxide device isillustrated, while in FIGS. 2C & 2D, a low VT-doped device, and highVT-doped device, respectively, are depicted. Each of these figuresrepresents a different device type as the term is employed in thepresent invention. Other logic circuits could be similarly implementedwith different device types such as illustrated for the inverters ofFIGS. 2A-2D. In one example, the device type refers to a transistordesign type having different functional characteristics. To enhancecorrelation in the aging effect, the ring oscillator sensor employed inFIG. 2 assimilates the device types used in the monitored digitalfunction block 205.

FIG. 3 depicts an alternate embodiment of a digital system 301 and amonitoring system 300, in accordance with an aspect of the presentinvention. In this embodiment, digital system 301 includes an observeddigital block 305 which comprises different logic types. The monitoringsystem 300 has a ring oscillator sensor 310 which has a logic circuitcomposition that mirrors the circuit composition of the observed digitalblock 305. In particular, the logic type composition in the observeddigital block 305 is assimilated in the ring oscillator sensor 310. Inthis example, a NOR logic circuit, NAND logic circuit and inverter logiccircuit are implemented within the observed digital block 305. Thus, thering oscillator sensor 310 assimilates percentages of these logiccomponents into the chain of logic devices defining the ring oscillatorsensor. By way of example, if 50% of the observed digital function block305 comprises NOR logic circuits, 25% NAND logic circuits and 25%inverter logic circuits, then the similar circuit compositionpercentages would, in one embodiment, be substantially repeated withinthe ring oscillator sensor. That is, the ring oscillator sensor would beimplemented with approximately 50% NOR logic circuits, 25% NAND logiccircuits and 25% inverter logic circuits. In this example, the logictypes are each implemented using device type “a”.

In FIG. 4, a digital system 401 is illustrated wherein an observeddigital function block 405 includes multiple logic types and multipledevice types. Thus, the associated monitoring system 400 has a ringoscillator sensor 410 which assimilates both the multiple logic typesand the multiple device types. More particularly, the observed digitalblock 405 is shown to include multiple NOR logic circuits, NAND logiccircuits and inverter logic circuits, each of which is implemented usinga different device type (i.e., device type “a”, device type “b” anddevice type “c”). The ring oscillator sensor has a circuit compositionwith logic and device type percentages that mirror the circuitcomposition of the observed digital block. In this case, 33% NOR logiccircuits, 33% NAND logic circuits and 33% inverter logic circuits, with50% of the NOR logic circuits being device type “a”, and 50% device type“b”, 50% of the NAND logic circuits being device type “a”, and 50%device type “b”, and 50% of the inverter logic circuits being devicetype “a”, and 50% device type “c”.

In FIG. 5, the digital system 501 is shown to comprise multiple digitalfunction blocks (including a digital function block 1 505, digitalfunction block 2 506 and digital function block k 507). Each digitalfunction block has an associated ring oscillator sensor (i.e., sensors510, 511 & 512, respectively). As in the above examples, each ringoscillator sensor 510, 511 & 512 assimilates the logic type(s) anddevice type(s) of the corresponding digital function block to bemonitored. In this way, aging of each ring oscillator sensor is tailoredto closely correlate to aging of the respective digital function blockbeing monitored.

FIG. 6 depicts a single digital function block 1 605 having multiplecircuit areas (i.e., circuit area a 606, circuit area b 607 and circuitarea c 608), within which the digital function block is implemented. Byway of example, a digital function block could be disposed within oracross a semiconductor device with different concentrations of circuitelements disposed at different locations within the semiconductordevice. Thus, aging of certain components at one location may bedifferent from aging of other components at another location within agiven digital function block (or within a given digital system). In thisexample, multiple ring oscillators 610, 611 & 612 are therefore providedeach having a composition which assimilates the circuit composition ofthe corresponding circuit area 606, 607 & 608 to be monitored. Theassimilation again is such that the composition of the correspondingring oscillator sensor closely mirrors the logic type and device typeemployed in the circuit area being monitored.

In addition to correlating aging of the ring oscillator sensor to thedigital system or digital function block based on logic and device type,powering and operation of the ring oscillator sensor can also be tied tothe digital system or digital function block being monitored. In FIG. 7,a digital function block 705 is illustrated wherein the monitoreddigital block is powered by a power supply VDD 720 which also powers themonitoring ring oscillator sensor 710. By powering the ring oscillatorsensor with the same power supply as used to drive the monitored digitalfunction block, better aging correlation can be achieved since thesensor experiences the same power level fluctuations as the digitalblock being monitored.

FIG. 8 depicts a similar concept wherein a digital system 801 includesmultiple digital function blocks (i.e., digital function block 1 805,digital function block 2 806 . . . digital function block k 807), whichare separately powered by power supplies VDD 1 820, VDD 821 . . . VDD k822. As shown, each associated ring oscillator sensor (i.e., ringoscillator sensor 1 810, ring oscillator sensor 2 811 . . . ringoscillator sensor k 812) is also powered by the same power supply asemployed to power the associated digital function block being monitored.Thus, if one of the digital function blocks is, for example, repeatedlypowered ON and OFF, the associated ring oscillator sensor for thatdigital function block is also repeatedly powered ON and OFF, whichbetter correlates the aging of the sensor to aging of the digitalfunction block. By way of example, a digital function block whichcomprises a floating point unit may be powered OFF when not in use inorder to conserve power. Thus, the associated ring oscillator sensor isalso powered OFF, in this embodiment, to better correlate aging of thering oscillator sensor to the floating point unit.

FIG. 9 depicts one embodiment of a digital block 905 which is activatedfrom a stand-by mode responsive to a request being received at thedigital block. In this embodiment, the ring oscillator sensor 910 issimilarly enabled only when the request is coming into the associateddigital block. Thus, transitions are occurring within the ringoscillator sensor only when a request is received and being acted uponat the digital block.

FIG. 10 illustrates a similar concept as shown in FIG. 9, only for adigital system 1000 comprising multiple digital function blocks (i.e.,digital function block 1 1005 . . . digital function block k 1006), eachof which receives its own computing request input. The associated ringoscillator sensors (ring oscillator sensor 1 1010 . . . . ringoscillator sensor k 1011) are electrically connected to also receive therespective request input as an enablement signal to enable the sensorwhen the associated digital function block is active.

FIG. 11 graphically depicts digital system and ring oscillator sensorlifecycles, in accordance with aspects of the present invention. Asillustrated, the lifecycle of the digital system begins with the digitalsystem having a maximum frequency of operation (F_(MAX)) above amanufacturer specified minimum acceptable maximum frequency of operation(F_(SPEC)). As time passes, the digital system ages and gradually themaximum frequency of operation of the digital system (F_(MAX)) degradesto a level at or below the manufacturer specified required maximumfrequency of operation (F_(SPEC)).

In accordance with an aspect of the present invention, a warningthreshold frequency (F_(WARN)) is provided. This predefined warningthreshold frequency (F_(WARN)) may be greater than or equal to themanufacturer specified required maximum frequency of operation of thedigital system (F_(SPEC)). In the lifecycle illustration of FIG. 11, thewarning threshold frequency of operation is above the manufacturerspecified minimum frequency of operation, and when the maximum frequencyof operation of the digital system (F_(MAX)) drops to or below thewarning threshold frequency of operation (F_(WARN)), a warning signal isgenerated by the control logic and sent, for example, to an operatingsystem of the digital system.

In this embodiment, the warning signal indicates that the maximumfrequency of operation of the digital system (F_(MAX)) may be slowerthan the manufacturer specified maximum frequency of operation(F_(SPEC)) in the near future. At this point, the warning signal mayalso be provided to a user of the digital system so than an appropriateprocedure, such as shutdown, can be taken. As explained further below,when the maximum frequency of operation of the digital system (F_(MAX))is at or below the warning threshold frequency of operation of thedigital system (F_(WARN)), the sampling rate for evaluating thefrequency of operation may also be increased to more accurately monitorthe digital system's status.

By correlating the composition and operation of the ring oscillatorsensor to the digital system, the lifecycle of the embedded ringoscillator sensor can be tailored to closely match that of the digitalsystem to be monitored. Thus, when the oscillation frequency of theembedded ring oscillator sensor described herein reaches the predefinedwarning threshold frequency (F_(WARN)), the warning signal can begenerated, which is assumed to be indicative of a reliabilitydegradation of the digital system itself. Also shown in this figure is aring oscillator sensor which is not as closely correlated to the digitalsystem aging. This alternate ring oscillator sensor may, for example, bepowered and active continuously, in contrast to the digital system(which may alternatively be powered ON/OFF and/or selectively activatedfrom a stand-by mode). In such a case, the alternative ring oscillatorsensor could provide an earlier warning signal that digital system agingis beginning to occur. Also, as noted, when the oscillation frequency ofthe ring oscillator sensor is at or below the warning thresholdfrequency of operation of the digital system (F_(WARN)), the samplingrate for determining the oscillation frequency of the ring oscillatorsensor may be increased to more accurately monitor the digital system'sstatus as described further below.

FIGS. 12-15 depict various alternate embodiments for control logic of amonitoring system, in accordance with an aspect of the presentinvention.

FIG. 12 depicts one embodiment for tracing oscillation frequency of aring oscillator sensor, in accordance with an aspect of the presentinvention. Upon power-up of the digital system 1200, the currentoscillation frequency of the ring oscillator sensor (F_(K)) is read at atime (T_(K)) 1210. In one embodiment, reading of the current oscillationfrequency of the ring oscillator sensor is synonymous with determiningthe current oscillation frequency of the ring oscillator sensor. Thelogic also fetches the previous oscillation frequency (F_(K−1)) of thering oscillator sensor at time T_(K−1) 1220. This previous oscillationfrequency can be retrieved from a trend database 1251, which isaccessible by the control logic. The difference (D_(K)) between theprevious oscillation frequency of the ring oscillator sensor and thecurrent ring oscillation frequency of the ring oscillator sensor isdetermined 1230, and the rate of change (R_(K)) in the difference iscalculated 1240. The measured oscillation frequency of the ringoscillator sensor (F_(K)), at time T_(K) is then recorded in the trenddatabase, along with the rate of change (R_(K)) in the difference(D_(K)) between oscillation frequencies of the ring oscillator sensor1250. After this, trend analysis 1260 may be performed, eithercommensurate with each periodic determination of the oscillationfrequency of the ring oscillator sensor, or at some other specifiedinterval.

FIG. 13 depicts one embodiment of a trend analysis approach wherein Nmost recent rates of change in the difference between measuredoscillation frequencies of the ring oscillator sensor are fetched 1300from the trend database 1251. From these values, a next rate of change(R′_(K+1)) in the difference between measured oscillation frequencies ofthe ring oscillator sensor is estimated 1310. This estimated next rateof change (R′_(K+1)) in the difference between measured oscillationfrequencies of the ring oscillator sensor can be determined employingconventional linear model estimation, such as a linear order model 1320,wherein a linear prediction is made from the previous N rate of changedeterminations. Alternatively, historical aging information obtainedfrom a ring oscillator aging database 1315 can be employed. By way ofexample, the historical aging information may contain informationgathered through conventional burn-in testing of the ring oscillatorsensor. Alternatively, historical aging information could be derivedfrom measuring aging of other ring oscillator sensors of the particulartype as the current ring oscillator sensor being evaluated. Dependingupon the ring oscillator sensor, this historical aging information mayprovide a more accurate estimate of the next rate of change in thedifference between oscillation frequencies of the ring oscillator sensorthan a linear progression model.

In the protocol of FIG. 13, the next oscillation frequency of the ringoscillator sensor (F′_(K+1)) is estimated 1330, after which the logicdetermines whether the estimated oscillation frequency (F′_(K+1)) isless than the predefined warning threshold frequency (F_(WARN)) 1340. Ifso, then a warning signal is generated 1350, which completes trendanalysis 1360. Assuming that the estimated next oscillation frequency ofthe ring oscillator sensor (F′_(K+1)) is greater than the predefinedwarning threshold frequency (F_(WARN)), then no warning signal isgenerated, and trend analysis is finished 1360.

The logic of FIG. 14 is similar to the logic of FIG. 13, with theexception that the next oscillation frequency of the ring oscillatorsensor (F′_(K+1)) is estimated directly from N prior saved measuredoscillation frequencies of the ring oscillator sensor. Specifically, themost recent N prior saved measured oscillation frequencies of the ringoscillator sensor are fetched 1400 from the trend database 1251, andfrom these values, the next oscillation frequency of the ring oscillatorsensor (F′_(K+1)) is estimated 1410 using, for example, linear N-ordermodel analysis 1320 or historical aging information obtained from a ringoscillator aging database 1315. If the estimated next oscillationfrequency of the ring oscillator sensor (F′_(K+1)) is less than thepredefined warning threshold frequency (F_(WARN)) 1420, then a warningsignal is generated 1430, thereby completing trend analysis 1440. Nowarning signal is generated if the estimated next oscillation frequencyof the ring oscillator sensor is above the warning threshold frequency.

FIG. 15 depicts a further embodiment of a trend analysis approachwherein N most recent rates of change in the difference between measuredoscillation frequencies of the ring oscillator sensor are fetched 1500from the trend database 1251. From these values, a next rate of change(R′_(K+)) in the difference between measured oscillation frequencies ofthe ring oscillator sensor is estimated 1510. This estimated next rateof change (R′_(K+)) in the difference between measured oscillationfrequencies of the ring oscillator sensor can be determined employingconventional linear model estimation, such as a linear N-order model1320, or alternatively, historical aging information obtained from aring oscillator aging database 1315. The estimated next rate of change(R′_(K+)) is then compared against an acceptable rate of changethreshold (R_(WARN)) for the digital system 1520. If the estimated nextrate of change is greater than the acceptable rate of change threshold,then a warning signal is generated 1530, which completes trend analysis1540. Assuming that the estimated next rate of change in the differencebetween measured oscillation frequencies of the ring oscillator sensoris below the acceptable rate of change threshold for the digital system,trend analysis is finished 1540.

FIG. 16 depicts one embodiment for analyzing and dynamically adjustingthe sampling period employed by the control logic in periodicallyevaluating the oscillation frequency of the ring oscillator sensor.

In FIG. 16, an approach is presented for determining a next time inwhich to sample the oscillation frequency of the ring oscillator sensor.In this approach, the most recently determined rate of change in thedifference between oscillation frequencies, as well as the most recentlymeasured oscillation frequency of the ring oscillator sensor, areretrieved 1600 from the trend database 1251 and used to estimate a timeinterval (T′_(K+)) for when an estimated oscillation frequency of thering oscillator sensor (F′_(K+)) will be equal to the warning thresholdfrequency of operation (F_(WARN)) 1610. This estimate can again beobtained either using historical aging information on the ringoscillator sensor (or alternatively, on the digital system type to whichthe ring oscillator sensor is correlated) which is retrieved, forexample, from historical aging database 1315, or by linear progressionanalysis using a linear N-order model 1320. The estimated sampling timeat which the estimated oscillation frequency of the ring oscillatorsensor will be at the predefined warning threshold frequency is thenused to determine an estimated sampling period to arrive at thatpredefined warning threshold frequency 1620. This estimated samplingperiod (P′_(K+)) is then compared against the previously employedsampling period (P_(K)) used in measuring the most recently obtainedoscillation frequency of the ring oscillator sensor 1630. If thepreviously employed sampling period is greater than the estimatedsampling period to arrive at the predefined warning threshold frequency(F_(WARN)), then the sampling time employed for the next measurement ofthe oscillation frequency of the ring oscillator sensor is the priorsampling time plus the estimated sampling period until the oscillationfrequency reaches the predefined warning threshold frequency 1640.Alternatively, if the previously employed sampling period is less thanthe estimated sampling period until the oscillation frequency reachesthe predefined warning threshold frequency (P′_(K+1)), then the nextsampling time is the prior sampling time plus the previously employedsampling period (P_(K)) 1660. Once the sampling time for the nextdetermination of the oscillation frequency of the ring oscillator sensoris determined, sampling period analysis is complete 1650.

As a further variation, the above-described control protocol may beintegrated with a control protocol such as described in commonlyassigned, co-pending U.S. patent application Ser. No. 11/733,318, filedApr. 10, 2007, and entitled “Monitoring Reliability of a DigitalSystem”, the entirety of which is hereby incorporated herein byreference. Briefly summarized, this co-pending application describes afurther technique for continually monitoring reliability, or aging, of adigital system and for issuing a warning signal if digital systemoperation degrades to or past a specified threshold. The techniqueincludes periodically determining a maximum frequency of operation ofthe digital system, and generating a warning signal indicative of areliability degradation of the digital system if at least one of: (i) ameasured or estimated maximum frequency of operation of the digitalsystem is below a warning threshold frequency of operation of thedigital system, wherein the warning threshold frequency is greater thanor equal to a manufacturer specified minimum required maximum frequencyof operation for the digital system; or (ii) a rate of change in thedifference between measured maximum frequencies of operation of thedigital system exceeds an acceptable rate of change threshold for thedigital system. By way of example, the warning signal may be generatedonly if both control protocols agree, that is, evaluation of oscillationfrequencies of the at least one ring oscillator sensor indicatesreliability degradation thereof, and evaluation of the maximum frequencyof operation of the digital system indicates reliability degradationthereof.

One or more aspects of the present invention can be included in anarticle of manufacture (e.g., one or more computer program products)having, for instance, computer usable media. The media has therein, forinstance, computer readable program code means or logic (e.g.,instructions, code, commands, etc.) to provide and facilitate thecapabilities of the present invention. The article of manufacture can beincluded as a part of a computer system or sold separately.

One example of an article of manufacture or a computer program productincorporating one or more aspects of the present invention is describedwith reference to FIG. 17. A computer program product 1700 includes, forinstance, one or more computer usable media 1702 to store computerreadable program code means or logic 1704 thereon to provide andfacilitate one or more aspects of the present invention. The medium canbe an electronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system (or apparatus or device) or a propagation medium.Examples of a computer readable medium include a semiconductor or solidstate memory, magnetic tape, a removable computer diskette, a randomaccess memory (RAM), a read-only memory (ROM), a rigid magnetic disk andan optical disk. Examples of optical disks include compact disk-readonly memory (CD-ROM), compact disk-read/write (CD-R/W) and DVD.

A sequence of program instructions or a logical assembly of one or moreinterrelated modules defined by one or more computer readable programcode means or logic direct the performance of one or more aspects of thepresent invention.

Advantageously, a data structure of readily accessible units of memoryis provided. By employing this data structure, memory access and systemperformance are enhanced (e.g., faster). The data structure includesdesignations (e.g., addresses) of one or more units of memory (e.g.,pages) that while in the data structure do not need address translationor any other test to be performed in order to access the unit of memory.This data structure can be used in any type of processing environmentincluding emulated environments.

Although various embodiments are described above, these are onlyexamples. For instance, one or more aspects of the present invention canbe included in environments that are not emulated environments. Further,one or more aspects of the present invention can be used in emulatedenvironments that have a native architecture that is different than theone described above and/or emulates an architecture other than thez/Architecture®. Various emulators can be used. Emulators arecommercially available and offered by various companies. Additionaldetails relating to emulation are described in Virtual Machines:Versatile Platforms For Systems and Processes (The Morgan KaufmannSeries in Computer Architecture and Design), Jim Smith and Ravi Nair,Jun. 3, 2005, which is hereby incorporated herein by reference in itsentirety.

Input/Output or I/O devices (including, but not limited to, keyboards,displays, pointing devices, DASD, tape, CDs, DVDs, thumb drives andother memory media, etc.) can be coupled to the system either directlyor through intervening I/O controllers. Network adapters may also becoupled to the system to enable the data processing system to becomecoupled to other data processing systems or remote printers or storagedevices through intervening private or public networks. Modems, cablemodems, and Ethernet cards are just a few of the available types ofnetwork adapters.

The capabilities of one or more aspects of the present invention can beimplemented in software, firmware, hardware, or some combinationthereof. At least one program storage device readable by a machineembodying at least one program of instructions executable by the machineto perform the capabilities of the present invention can be provided.

The flow diagrams depicted herein are just examples. There may be manyvariations to these diagrams or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order, or steps maybe added, deleted, or modified. All of these variations are considered apart of the claimed invention.

Although preferred embodiments have been depicted and described indetail herein, it will be apparent to those skilled in the relevant artthat various modifications, additions, substitutions and the like can bemade without departing from the spirit of the invention and these aretherefore considered to be within the scope of the invention as definedin the following claims.

1. A system for monitoring reliability of a digital system, the systemcomprising: at least one ring oscillator sensor implemented inassociation with the digital system for facilitating monitoringreliability thereof, the digital system including a circuit compositioncomprising at least one logic type, the at least one logic typecomprising at least one device type, and wherein the at least one ringoscillator sensor includes a circuit composition comprising one or moreof the at least one logic type or the at least one device type of thedigital system selected based on the circuit composition of the digitalsystem to correlate aging of the at least one ring oscillator sensor toaging of the digital system, the at least one ring oscillator sensoroutputting count signals; counter logic coupled to the at least one ringoscillator sensor for converting the count signals to an oscillationfrequency of the at least one ring oscillator sensor; and control logiccoupled to the counter logic for periodically evaluating oscillationfrequency of the at least one ring oscillator sensor and generating awarning signal indicative of reliability degradation thereof, and henceof the digital system, if at least one of: (i) a measured or estimatedoscillation frequency of the at least one ring oscillator sensor isbelow a warning threshold frequency for the digital system; or (ii) ameasured or estimated rate of change in a difference between measuredoscillation frequencies of the at least one ring oscillator sensorexceeds an acceptable rate of change threshold for the digital system.2. The system of claim 1, wherein the circuit composition of the digitalsystem comprises multiple logic types, and the circuit composition ofthe at least one ring oscillator sensor also comprises the multiplelogic types, wherein percentages of specific logic types of the multiplelogic types within the at least one ring oscillator sensor mirrorpercentages of specific logic types of the multiple logic types withinthe digital system.
 3. The system of claim 1, wherein the circuitcomposition of the digital system comprises multiple device types, andthe circuit composition of the at least one ring oscillator sensor alsocomprises the multiple device types, wherein percentages of specificdevice types of the multiple device types within the at least one ringoscillator sensor mirror percentages of specific device types of themultiple device types within the digital system.
 4. The system of claim1, wherein the circuit composition of the digital system comprisesmultiple logic types and multiple device types, and wherein the circuitcomposition of the at least one ring oscillator sensor also comprisesthe multiple logic types and the multiple device types of the digitalsystem, wherein percentages of specific logic types of the multiplelogic types and specific device types of the multiple device typeswithin the at least one ring oscillator sensor mirror percentages ofspecific logic types of the multiple logic types and specific devicetypes of the multiple device types within the digital system.
 5. Thesystem of claim 1, wherein the digital system comprises multiple digitalfunction blocks and multiple circuit areas, and wherein the systemfurther comprises multiple ring oscillator sensors, each ring oscillatorsensor being associated with at least one of the multiple digitalfunction blocks or multiple circuit areas of the digital system tomirror at least one of a logic type or a device type composition thereofand provide count signals useful in monitoring aging of the associatedat least one digital function block or circuit area of the digitalsystem.
 6. The system of claim 1, wherein the circuit composition of thedigital system comprises multiple logic types, the multiple logic typescomprising at least two of AND logic, NAND logic, OR logic, NOR logic,and INVERTER logic, and wherein the multiple logic types of the digitalsystem further comprise at least one device type, the at least onedevice type comprising one or more of a thin oxide device, a thick oxidedevice, a low VT-doped device or a high VT-doped device.
 7. The systemof claim 1, wherein the digital system and the at least one ringoscillator sensor are powered by a common power supply.
 8. The system ofclaim 1, wherein the digital system comprises multiple digital functionblocks, and wherein the system further comprises multiple ringoscillator sensors, each ring oscillator sensor being embedded within arespective digital function block of the multiple digital functionblocks, and wherein each digital function block and associated ringoscillator sensor are powered by a common power supply.
 9. The system ofclaim 1, wherein the digital system further comprises at least onedigital function block, and the at least one ring oscillator sensor isassociated with the at least one digital function block, and wherein theat least one ring oscillator sensor further comprises an enable inputelectrically coupled to an input of the associated at least one digitalfunction block, the at least one ring oscillator sensor being enabledwith receipt of a request at the associated at least one digitalfunction block.
 10. The system of claim 1, wherein the control logicemploys multiple determined oscillation frequencies of the at least onering oscillator sensor in estimating a next oscillation frequency of theat least one ring oscillator sensor, and wherein the generating of thewarning signal comprises generating the warning signal if the estimatednext oscillation frequency of the at least one ring oscillator sensor isbelow the warning threshold frequency.
 11. The system of claim 1,wherein the control logic periodically determines a rate of change in adifference between measured oscillation frequencies of the at least onering oscillator sensor, and employs multiple determined rates of changebetween measured oscillation frequencies of the at least one ringoscillator sensor in estimating a next rate of change employing one of alinear model estimation or a historical aging data for the at least onering oscillator sensor, and wherein the control logic estimates a nextoscillation frequency of the at least one ring oscillator sensoremploying the estimated next rate of change in the difference betweenmeasured oscillation frequencies of the at least one ring oscillatorsensor, and wherein generating the warning signal comprises generatingthe warning signal if the estimated next oscillation frequency of the atleast one ring oscillator sensor is below a warning threshold frequencyfor the digital system.
 12. The system of claim 1, wherein the controllogic further dynamically adjusts a sampling period employed in theperiodically evaluating oscillation frequency of the at least one ringoscillator sensor, the dynamically adjusting comprising: estimating atime interval from a most recent determination of oscillation frequencyof the at least one ring oscillator sensor to the at least one ringoscillator sensor reaching the warning threshold frequency; employingthe estimated time interval in setting a next sampling period fordetermining the oscillation frequency of the at least one ringoscillator sensor; determining whether the next sampling period is lessthan a previous sampling period employed in the periodically evaluatingoscillation frequency of the at least one ring oscillator sensor; and ifso, increasing the sampling period to increase the sampling rate of theperiodically evaluating oscillation frequency of the at least one ringoscillator sensor.
 13. A system for monitoring reliability of a digitalsystem, the system comprising: at least one ring oscillator sensorembedded within a digital system for facilitating monitoring reliabilitythereof, the digital system including a circuit composition comprisingat least one logic type, and at least one device type employed in the atleast one logic type, and wherein the at least one ring oscillatorsensor includes a circuit composition at least partially mirroringpercentages of the at least one logic type and the at least one devicetype employed in the circuit composition of the digital system, whereinaging of the at least one ring oscillator sensor is correlated to agingof the digital system, the at least one ring oscillator sensoroutputting count signals; counter logic coupled to the at least one ringoscillator sensor for converting the count signals to an oscillationfrequency of the at least one ring oscillator sensor; and control logiccoupled to the counter logic for periodically evaluating oscillationfrequency of the at least one ring oscillator sensor and generating awarning signal indicative of reliability degradation thereof, and henceof the digital system, if at least one of: (i) a measured or estimatedoscillation frequency of the at least one ring oscillator sensor isbelow a warning threshold frequency for the digital system; or (ii) ameasured or estimated rate of change in a difference between measuredoscillation frequencies of the at least one ring oscillator sensorexceeds an acceptable rate of change threshold for the digital system.14. The system of claim 13, wherein the digital system and the at leastone ring oscillator sensor are powered by a common power supply, logicand device type circuit composition of the at least one ring oscillatorsensor mirrors percentages of logic and device types employed in thedigital system, and the at least one ring oscillator sensor is enabledonly when the digital system is active, thereby facilitating correlatingaging of the at least one ring oscillator sensor to aging of the digitalsystem.
 15. A method of monitoring reliability of a digital system, themethod comprising: obtaining at least one ring oscillator sensorembedded within a digital system for facilitating monitoring reliabilitythereof, the digital system including a circuit composition comprisingat least one logic type, the at least one logic type comprising at leastone device type, and wherein logic and device type compositionpercentages for the at least one ring oscillator sensor mirror circuitcomposition percentages of one or more of the at least one logic type orthe at least one device type within the digital system to correlateaging of the at least one ring oscillator sensor to aging of the digitalsystem, the at least one ring oscillator sensor outputting countsignals; converting the count signals of the at least one ringoscillator sensor to an oscillation frequency of the at least one ringoscillator sensor; and periodically evaluating oscillation frequency ofthe at least one ring oscillator sensor, and generating a warning signalindicative of a reliability degradation thereof, and hence of thedigital system, if at least one of: (i) a measured or estimatedoscillation frequency of the at least one ring oscillator sensor isbelow a warning threshold frequency for the digital system; or (ii) ameasured or estimated rate of change in a difference between measuredoscillation frequencies of the at least one ring oscillator sensorexceeds an acceptable rate of change threshold for the digital system.16. The method of claim 15, wherein the circuit composition of thedigital system comprises multiple logic types and multiple device types,and wherein the circuit composition of the at least one ring oscillatorsensor also comprises the multiple logic types and the multiple devicetypes of the digital system, wherein percentages of specific logic typesof the multiple logic types and specific device types of the multipledevice types within the at least one ring oscillator sensor mirrorpercentages of specific logic types of the multiple logic types andspecific device types of the multiple device types within the digitalsystem.
 17. The method of claim 15, wherein the digital system comprisesmultiple digital function blocks and multiple circuit areas, and whereinthe obtaining further comprises obtaining multiple ring oscillatorsensors, each ring oscillator sensor being associated with at least oneof the multiple digital function blocks or multiple circuit areas of thedigital system to mirror at least one of a logic type or a device typecomposition thereof and provide count signals useful in monitoring agingof the associated at least one digital function block or circuit area ofthe digital system.
 18. The method of claim 15, wherein the digitalsystem and the at least one ring oscillator sensor are powered by acommon power supply, and wherein the method further comprises enablingthe at least one ring oscillator sensor only when the digital system isactive, thereby facilitating correlating aging of the at least one ringoscillator sensor with aging of the digital system.
 19. The method ofclaim 15, wherein the digital system further comprises multiple digitalfunction blocks, and the obtaining comprises obtaining multiple ringoscillator sensors, each ring oscillator sensor being embedded within arespective digital function block of the multiple digital functionblocks, and wherein each digital function block and associated ringoscillator sensor share a power supply and the associated ringoscillator sensor is only enabled when the digital function block isactive.
 20. The method of claim 15, wherein the circuit composition ofthe digital system comprises multiple logic types, the multiple logictypes comprising at least two of AND logic, NAND logic, OR logic, NORlogic, and INVERTER logic, and wherein the multiple logic types of thedigital system further comprise at least one device type, the at leastone device type comprising one or more of a thin oxide device, a thickoxide device, a low VT-doped device, or a high VT-doped device.